The present invention relates to an optical pick-up device and an objective lens used therein, and in particular, to an optical pick-up device, having therein a light source unit in which a plurality of light sources having different wavelengths are arranged on a single base board, and performing recording and/or reproduction of information for different optical information recording media, and an objective lens used therein.
There has been developed, for example, an interchangeable optical pick-up device which is provided with two light sources to perform the recording or reproduction of information for DVD, and to perform the recording or reproduction of information for CD. In the optical pick-up device mentioned above, there are various strict requirements such as a low price setting and miniaturization of the construction.
Incidentally, in the case of the reproduction of information stored in DVD, an optical magnification which is requested to a light-convergent optical system of the optical pick-up device is about − 1/7 to −⅙, however, in the case of the recording or reproduction of information for CD, the optical magnification which is requested to the light-convergent optical system is about −¼ to obtain a large amount of light. However, to compose the light-convergent optical system having the different optical magnifications separately cannot perform the miniaturization and low price setting for the optical pick-up device.
In the prior art, therefore, the above-mentioned different optical magnifications are attained by the following manners, that is, two optical light sources are arranged in a single light-convergent optical system separately, instead of using the different exclusive light-convergent optical system (an optical pick-up optical system) for CD and DVD respectively, and the light source for the recording of information or reproduction of information for CD is arranged closer to the light-convergent optical system than the light source for the reproduction of information for DVD, and further, in an optical path, there is provided a lens through which a only light flux for the recording or reproduction of information for CD passes.
On the contrary, when the recording on an optical disk is not regarded as important, it is not indispensable to change the optical magnification, because to obtain the light amount is not a purpose.
Further, for the interchangeable optical pick-up device, there has been developed an optical pick-up device used for the so-called combo drive, which is provided with the exclusive light sources for DVD and CD respectively, to reproduce information stored in DVD, and to record or reproduce information for CD.
On the other hand, there is developed the light source unit in which a plurality of light sources having different wavelengths are arranged on the single base board, and to use this light source unit can perform the simplification of the construction and the low cost setting of the optical pick-up device. However, in case of using this light source unit, there is a problem that the different magnifications must be established for CD and DVD respectively, though the light fluxes pass through the same optical path, because the distances from both of the light sources to the light-convergent optical system are the same.
That is, when the light source unit stated above is used for the optical pick-up device used for the above-mentioned combo drive, the optical magnification that is suitable for the reproduction for DVD is unsuitable for the reproduction and recording for CD.
Incidentally, regarding a thickness of a protection base board (transparent base board) provided on an information recording surface of the above-mentioned each optical information recording medium, the thickness for DVD is 0.6 mm, and that for CD is 1.2 mm.
When the above-mentioned light source unit is used, the distances from the light source to the information recording surfaces of the optical information recording media are nearly the same, and due to this, in the case of recording or reproducing for CD having the thicker protection base board, it is difficult to obtain a sufficient working distance (an operation distance) from an objective lens to the surface of the optical information recording medium, which is a problem.
The invention has been achieved in view of the above-mentioned problem, and the object of the invention is to provide an optical pick-up device and an objective lens used therein which can record or reproduce information for the different optical information recording media having the different thickness of the protection base boards, by using the light source unit in which a plurality of the light sources having the different wavelengths are arranged on the single base board, and by using the single light-convergent optical system.
Especially, in the optical pick-up device which can perform only the reproduction of information for the one optical information recording medium, and can perform the reproduction and recording of information for the other optical information recording medium, there is a request to make the amount of light larger for the recording, accordingly, it is preferable if the focal length can be established optionally for the optical disk used for the recording.
Further, the object of the invention is to provide an optical pick-up device which is able to obtain the working distance for the desired optical information recording medium.